Why do we sleep? Questions of biology don't get much more fundamental than that. This year, neuroscientists took what looks like a major stride toward an answer. Most researchers agree that sleep serves many purposes, such as bolstering the immune system and consolidating memories, but they have long sought a "core" function common to species that sleep. By tracking colored dye through the brains of sleeping mice, scientists got what they think is a direct view of sleep's basic purpose: cleaning the brain. When mice slumber, they found, a network of transport channels through the brain expands by 60%, increasing the flow of cerebral spinal fluid. The surge of fluid clears away metabolic waste products such as β amyloid proteins, which can plaster neurons with plaques and are associated with Alzheimer's disease.
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